Production of potato granules

ABSTRACT

Dehydrated potato granules are prepared by cooking, mashing, freezing, predrying, granulating and finally drying the resulting product to about 7% moisture.

This invention relates to improved dehydrated potato granules and to aprocess for the preparation of these granules.

The process now generally in use for the production of pre-cookeddehydrated potato pieces is commonly referred to as the "add-back"process. This name derives from the main distinguishing feature of theprocess which is that a large proportion of the final dried potatoproduct is recycled and mash-mixed with freshly cooked potatoes. Varioussubsidiary steps, each essential to the particular process, as, forexample, pre-cooking or conditioning, may intervene at one point oranother. At any rate, the recycling and mash-mix technique serves toreduce the moisture content of the mixture down to about 35% and tosubdivide the cooked potato tissue into fine granules. The mixture issubsequently dried in dryers which might typically consist of anair-lift dryer and/or fluidized bed dryer.

In the above described process, about 85-90% of the dried product isrequired to be "added-back" for mixing with freshly cooked potatoes.Hence, only 10-15% of the total process throughput is packed as a finalproduct.

"Add-back" processes are marked by a number of distinct disadvantages.Perhaps the most obvious of these is that, due to the high recycle rate,the processing equipment is required to be, relatively very large. Inaddition, any given quantity of potatoes passing through the system issubjected to mash-mixing, conditioning, final granulation and any otheroperations which may be present in the system, on an average of 6-10times before packing. This also is a direct result of the high recyclerate. These repetitive treatments may result in cell damage due both tomechanical breakage and to repeated exposure to high temperature. Commonmanifestations of such damage are that the reconstituted product mayhave an undesirable gluey texture and a scorched or burnt flavour, andbe of reduced nutritional value particularly with respect to more heatsensitive components, in particular ascorbic acid (Vitamin C).

A further troublesome problem stemming from high recycle rate is that abatch of potatoes that was defective before entering the process, or aquantity that was rendered defective by poor control, etc., willcontinue to contaminate new batches for a lengthy period.

In addition to problems associated with recycle, add-back processesoften involve a conditioning step following mix-mashing wherein thepotato is subjected to a lengthy tempering period at a temperature suchthat microorganism activity and/or undesirable chemical reactions mayoccur resulting in such problems as discoloration.

With so many problems inherent in these systems, attempts have been madeto devise processes which would improve the processing technique byemploying a straight-through method with the elimination recycling.Various systems have been proposed each with its distinguishingcharacteristics and also with a number of common features. Obviously,all will require steps such as cooking, drying and mashing. However, thearrangement of the common steps, the addition of new ones, the deletionof unwanted ones and the conditions under which each step is carriedout, all serve to clearly set off each process from the others. In eachcase a new process is designed to improve the efficiency of operationwhile maintaining or improving the desirable characteristics of theproduct. On the other hand, each process proposed to date exhibits oneor more undesirable characteristics, for example, in its effect on thephysical or nutritional qualities of the product.

The process of the present invention provides a new method wherein theoverall effect of the process is to cause very little damage to eitherthe physical or nutritional properties of the product. In addition, theprocess results in very little discard or recycling. The presentinvention provides dehydrated potato granules having improvedcharacteristics and a process for the preparation of the granules whichcomprises the steps of preparing the potatoes for cooking, cooking thepotatoes, mashing the cooked potatoes at a temperature not less thanabout 160° F, cooling and freezing the potato mash, thawing the mash,predrying the thawed mash to reduce the moisture content to a range ofabout 35 - 42%, granulating the predryed potatoes, and drying the potatogranules to reduce the moisture content to not more than about 7%.

It is an advantage in this type of process that the time between thawingand the beginning of predrying be kept as short as possible. This isfacilitated by carrying out the mashing step before freezing. As thematerial freezes, pure ice crystals form in the intercellular spaces andhence the intercellular liquid becomes more concentrated. The result ofthis is that the moisture inside the cells passes through the cell wallsby osmosis. Hence, when the material is thawed most of the liquid isbetween the cells. It follows that if predrying is begun immediately,the quantity of water that will diffuse back into the cells is kept to aminimum. This is very useful since the moisture can be evaporated muchmore quickly outside the cells than within. Up to 80% of the initialwater in the potato can be removed in this manner.

The freezing and thawing steps are of importance also since the effectof the steps is to toughen the potato cells so that they can withstandmild mechanical force during predrying and relatively rigorousmechanical force during the short granulation step, without sustainingmore than minor damage.

The product has a high proportion of -60 mesh particles, normally notless than 85%, and very little discard, normally less than 1%. Theintermediate size particles which normally comprise not more than 15% ofthe product can be recycled into the predrying step without any illeffect.

Irrespective of the types of potatoes used in the process, thereconstitution ratio of the product is substantially constant at about1:4 (granules:water). The reconstituted product gives firm, mealy mashedpotatoes with flavour and texture resembling very closely those offreshly cooked and mashed potatoes.

During processing, the potato material is kept normally at lowtemperature with no lengthy tempering period. Hence the produce would beexpected to retain most of the original nutritional value of thepotatoes.

The operation of the process may be conveniently explained by referenceto the various processing steps as illustrated in FIG. 1.

In step 1 the potatoes are prepared for cooking. This may comprisepeeling, cutting, washing, together with sulphiting, if desired. Whilesulphiting is not essential to the success of the process, it mayadvantageously be added at this stage as is the normal industrialpractice in such processes. If sulphiting is carried out, it ispreferably done by soaking the washed potato pieces for about 5 minutesin a sodium bisulphite solution. This sulphite addition inhibitsenzymatic browning before cooking and also contributes to the totalsulphite in the final product.

Step 2 is the cooking step wherein the potatoes are cooked in anyconventional manner such as in an atmospheric steam cooker or in water.

Step 3 in the process comprises mashing the potatoes. This step iscarried out as soon as possible after cooking and while the potatopieces are still at a temperature above about 160° F. When mashing iscarried out above this temperature, the proportion of broken cells issmall, and the amount of free starch released from broken cells is wellbelow the level that would yield an undesirably doughy or pasty producton reconstitution. The mashing operation can be advantageously carriedout in an enclosed system in order to reduce evaporative cooling of thematerial. The mashing may be carried out using any conventionalapparatus, provided only that the mashing is complete and that it doesnot cause excessive cell damage.

During mashing additional sulphite may be added to bring the totalsulphite in the final product up to some value required by thesubsequent user of the granules. The total sulphite in the productaffects shelf life by inhibiting non-enzymatic browning and microbialgrowth. Hence, for example, the U.S. military authorities require asulphite level of 400 ppm in the product in order that it be acceptableto them.

Also at the mashing stage, a surfactant may be added. Addition at thisstage ensures a good distribution through the product. A preferredamount of surfactant is from about 0.15% to about 0.3% of the weight ofthe cooked potato. The surfactant has the effect of improving thetexture of the product by making it less gluey or sticky. This is due tothe surfactant binding the water soluble free starch and pecticsubstances which are present in the cooked potato or are released fromany cells which may be broken during processing. Surfactant couldalternatively be added just prior to predrying.

In addition, certain other additives may be added in the mashing stage,if desired. These include chelating agents such as ethylene diaminetetraacetic acid (EDTA); or other substances such as sodium gluconateand sodium acid pyrophosphate, all of which help to prevent theappearance of after-cooking darkening. There may also be added, ifdesired, antioxidants such as butylated hydroxyanisole, butylatedhydroxytoluene, propyl gallate, citric acid and/or other antioxidants,all of which help to prevent the occurrence of oxidative rancidityduring storage of the dry product.

Following mashing in step 4 freezing is carried out. If desired, the hotmash may be cooled before freezing in a separate step. Such a procedureis advantageous in that it reduces the load on the freezing apparatus.The potato mash is preferably frozen using circulating air at atemperature between about 0° and about -40° F.

Step 5 comprises thawing the potatoes. The operation is preferablycarried out using circulating air at a temperature between about 40° andabout 90° F, and continued only until the potatoes have just thawedcompletely. The potato temperature during this step should preferably bekept below about 50° F. Any conventional apparatus will be applicable tothis step depending on quantity of potatoes to be thawed and otherfactors dictated by the particular application. The potatoes are thenimmediately transferred to the predrying step. The predrying step may,if desired, be started before the potato mash is completely thawed.

In step 6, predrying, the potatoes are partially dried to reduce themoisture content to a range of about 35% to about 40%. This operationcan be conveniently carried out in a stirred bed or similar type ofdryer. As indicated above, by virtue of the freezing and thawingoperations, the potatoes contain a large proportion of moisture in theintercellular spaces. The mass can be dried quickly under a hot airstream while being slowly stirred. For example, while being stirred at amaximum lineal rate of 30-100 feet per minute the potatoes may besubjected to hot air at a temperature of 100°-300° F having asuperficial velocity of 250-500 feet per minute, for a period of 10-60minutes. Under such conditions the temperature of the potatoes does notreach undesirable levels nor is there excessive cell damage.Furthermore, the rate of predrying is maintained throughout in theconstant rate period. The term "constant" rate is well known in the artof food drying, and simply describes a drying period in which the rateof moisture loss is more or less constant over an appreciable portion ofthe total time involved. At the end of the predrying period the moisturecontent of the potatoes enters a range of from about 35% to about 42%,and the potato cells are well separated into single units or cellaggregates. In this range the cells are most resistant to physicaldamage, and are therefore in the most desirable condition forgranulation.

An air classification apparatus can be advantageously used during thepredrying and granulation stages. By the proper choice of air velocitythe potato particles will be carried over continuously into a subsequentprocessing step at just the desired conditon of the particles. Hence,the air velocity in the predrying step can be adjusted to carry over thepotato particles into the granulation step just when they are in thecritical range. In other words, the particles reach the granulator whenthe cells are most resistant to physical damage.

Granulation is effected in step 7 of the process. In this step it isdesirable that the moisture content of the particles remain within thecritical range for the entire operation, since, as indicated above, lessdamage will result to the physical structure of the cells under theseconditions. This step is therefore carried out relatively quickly. Theoperation can be carried out effectively in a stirred-bed fluidizingunit using a high rate of stirring and a low air velocity. For example,while being stirred at a maximum lineal rate of 1000-1800 feet perminute, the potatoes may be subjected to air flow at a temperature of100°-150° F and a superficial velocity of 20-60 feet per minute for aperiod of preferably 5-15 minutes duration. A preferred air velocity is35-45 feet per minute and a preferred maximum lineal stirrer speed isabout 1400 feet per minute.

This granulation rate allows little change in moisture content so thatthe particles remain within the critical range.

The granules leaving the granulator usually consist of more than 85%fine powder of smaller than 60 mesh and with less than 1% of discard.

Step 8, the final processing step, drying, can be carried out in anyconventional manner. For example, a batch fluidized bed may be usedsuccessfully. Other suitable types of apparatus are direct rotary dryer,screw conveyor dryer, or vibrating tray dryer.

The product thus produced shows an improved broken cell count and animproved Blue Value Index. The broken cell count is a measure of thepercentage of broken cells in the product. This may be obtained byexaminating a suspension of iodine-stained cells under a microscope at40 to 100X magnification, using the method of Reeve and Notter, FoodTechnol. 13 (10) 574-577 (1959), but with the addition of two drops ofstandard iodine solution to the suspension of cells on the microscopeslide. The iodine solution is prepared by the method of Williams andFegal, Cereal Chem. 46, 56-62 (1969). The product of the presentinvention is thus found to have a broken cell count of not more thanabout 2%.

The Blue Value Index (BVI) is a measure of the free starch content ofthe product and may be obtained by the method of Mullins et al, FoodTechnology, v. 9, p. 393 (1955). BVI values for the product of thepresent invention have been obtained down to about 110 BVI units,without the addition of a surfactant. Small amounts of a surfactantmaterial substantially reduce the BVI. For example, the addition of 0.1%by weight of glycerolmonostearate at the mashing stage may reduce theBVI to approximately 60.

What I claim as my invention is:
 1. A method for the preparation ofdehydrated potato granules which comprises subjecting potatoes to thefollowing steps in order:a. preparing the potatoes for cooking; b.cooking the potatoes; c. immediately mashing the cooked potatoes at atemperature of at least 160° F; d. cooling and freezing the potato mash;e. thawing the mash; f. immediately predrying the thawed mash byevaporation to reduce the moisture content to a range of about 35 - 42%;thereby producing potato cells which are separated into single units orcell aggregates g. thereafter granulating the predryed mash underconditions such that the moisture content remains within said 35-42%range during the entire granulating operation; and h. drying the potatogranules at a temperature and for a period of time sufficient to reducethe moisture content to not more than about 7%.
 2. The method of claim 1comprising the additional step of sulphiting the potatoes after step (a)and before step (b).
 3. The method of claim 2 wherein the sulphitingstep comprises soaking the washed potatoes in a sodium bisulphitesolution.
 4. The method of claim 1 comprising the additional step ofsulphiting the potatoes during the mashing step.
 5. The method of claim1 comprising the additional step of adding a surfactant to the potatoesduring the mashing step or just prior to the predrying step.
 6. Themethod of claim 5 wherein the amount of surfactant added is betweenabout 0.15% and about 0.3% of the weight of the cooked potato.
 7. Themethod of claim 1 wherein the freezing step is carried out at atemperature of from about 0° to about -40° F.
 8. The method of claim 1wherein the hot mash is cooled by forced circulation of air beforefreezing.
 9. The method of claim 1 wherein the thawing step is carriedout at an air temperature of from about 40° to about 90° F.
 10. Themethod of claim 9 wherein the temperature of the potato mash duringthawing is not permitted to rise above about 50° F.
 11. The method ofclaim 1 wherein the predrying step is begun before the potato mash iscompletely thawed.
 12. The method of claim 1 wherein the predrying iscarried out in a stirred bed dryer wherein the mash is simultaneouslystirred and contacted with a hot air stream.
 13. The method of claim 12wherein the maximum lineal speed of the stirrer is 30-100 feet perminute.
 14. The method of claim 12 wherein the temperature of the hotair stream is 100°-300° F.
 15. The method of claim 14 wherein thesuperficial velocity of the hot air stream is 250-500 feet per minute.16. The method of claim 12 wherein the granulating step is carried outin a stirred bed fluidizing unit using a higher rate of stirring, lowertemperature and lower air velocity, as compared to the predrying step.17. The method of claim 16 wherein the temperature of the air stream is100°-150° F.
 18. The method of claim 16 wherein the maximum lineal speedof the stirrer is 1000-1800 feet per minute.
 19. The method of claim 16wherein the superficial velocity of the air stream is 20-60 feet perminute.
 20. The method of claim 18 wherein the maximum lineal speed ofthe stirrer is 1400 feet per minute.
 21. The method of claim 19 whereinthe superficial air velocity is 35-45 feet per minute.
 22. The method ofclaim 1 wherein the granules leaving the granulator consist of at least85% fine powder of smaller than 60 mesh.
 23. The method of claim 16,wherein the air velocity of the hot air stream employed in the predryingstep is adjusted so that mash particles will be carried overautomatically from the stirred bed dryer of the predrying step to thestirred bed fluidizing unit of the granulating step when the moisturecontent of the mash particles has been reduced to within the range ofabout 35 to 42%.